The present invention relates to direction heading sensors or compasses. More specifically, the present invention relates to electronic compasses for automobiles which determine geographic orientation based upon electronic circuit elements which are sensitive to alignment within the Earth's magnetic field.
In general, compasses determine geographic orientation with reference to the Earth's magnetic field, which generally runs north and south for most populated parts of the Earth. While the magnetic poles of the earth are not located in the same position as the Earth's geographic poles, the magnetic north pole is generally an acceptable approximation to the geographic north pole. Conventional compasses employ a freely movable, magnetically sensitive member which aligns itself with the Earth's magnetic field and points towards the Earth's magnetic north pole.
Electronic compasses, on the other hand, determine geographic orientation without using a magnetically sensitive, movable member. In general, electronic compasses determine geographic orientation by simultaneously measuring the magnitude of the earth' magnetic field in at least two different directions which are related to each other at a known angle. Because the measurements are dependant upon the orientation of the measuring device, they can be used to determine the deviation of the compass's orientation from magnetic north, which is an approximation of geographic north. As an example, if the two measurements are taken at an angle of 90.degree. with respect to each other, the angular deviation of the compass's orientation with respect to magnetic north can be computed by taking the arctangent of the ratio of the two measurements. Because any number has more than one arctangent, further techniques must be used to determine which is the correct angle. For example, if the two measurements are equal, their ratio will be one, and the arctangent will be 45.degree. and 225.degree. (among other multiples of 360.degree.). Resolving which of these two possibilities is correct is accomplished by techniques which will be more fully described herein.
Measuring the magnitude of the Earth's magnetic field in a particular direction is generally accomplished through the use of materials whose magnetic permeability depends upon the magnitude of an external magnetic field, such as the Earth's. Thus, as these materials are rotated in the presence of the Earth's external magnetic field, their magnetic permeability will change due to the changing strength of the Earth's magnetic field when measured in different directions. Furthermore, because the inductance of a coil is dependent upon the magnetic permeability of its core material, a change in inductance can be detected by using this material as the core of a coil. By sensing these inductance changes, which can be electronically measured by using an oscillator circuit or the like, the magnitude of the Earth's magnetic field in a particular direction can be electronically measured. The changes in inductance are typically measured by employing an oscillator circuit whose frequency of oscillation is dependent upon the inductance. As the inductance changes, both frequency and amplitude changes will occur in the oscillating signal. Either or both of these changes can be the basis for determining the magnitude of the Earth's magnetic field in that particular direction.
There are several problems which arise in electronic compasses operating according to the above described principles. These include both the cost and complexity of the circuitry necessary to analyze the changes in frequency or amplitude of the oscillating signal. Additional problems arise due to the fact that the permeability of the core material tends to drift over time and with temperature changes. It is therefore an object of a present invention to provide a digital electronic compass which is both inexpensive to manufacture and operates with a minimum of complexity. It is another object the invention to provide a digital electronic compass which is unaffected by the permeability drift of the core material. It is still another object of the present invention to provide an electronic compass powered by a constant current source which is substantially unaffected by changes in ambient temperature. These and other objects of the invention will be apparent upon further review of this specification when read in light of the appended drawings.